A new "Slant" on things!

I have a 77 D200, unknown mileage, ex-military, crew cab. Bought for about $500 in 91, beat the snot out of it and will never give it up as long as I can fix it! Moved from OK. to NE. in 94, had a 4000 lb slide in camper in the bed, 2 layers of fruit boxes packed in inside, all packed full of books, and a tandem axle trailer hooked on loaded to about 6000 lb. averaged 11 mpg at 65-70 mph. All this with the original slant six!
Don't tell me the slant is a weak engine! On a return trip to OK. it burned a hole in #6 piston big enough to stick my thumb through (vacuum leak) and it still went 150 miles pulling an empty trailer at 65 mph. I have people tell me to junk the truck and get a new one, that's NEVER going to happen!